I think it all depends on your current level of knowledge. If you already know and understand a decent amount of EJB, and can *purchase* only one book (given that the spec and tutorial from Sun are free, so I won't count those --
you should get them!) then I would choose Richard's book (the O'Reilly EJB book) rather than ours, unless your goal is to pass the exam, fairly soon, in which case Richard's book will make it more difficult to do that in a short period of time.
However, if you do NOT know EJB well at all, then I would say start with our book if you want to come up to speed in the quickest way possible. You can CERTAINLY do that with Richard's book, but ours will get you to a solid foundation more quickly, while his can take your further, but it will take you more time to really *get* the foundation knowledge at a deep level. We spend pages and pages on things he covers in one sentence, while he covers more topics than we do.
Hope that helps
cheers,
Kathy